This application claims priority to an application entitled xe2x80x9cMethod for Optimally Allocating Channel Power in W-CDMA WLL systemxe2x80x9d filed in the Korean Industrial Property Office on Mar. 15, 1999 and assigned Serial No. 99-8557.
1. Field of the Invention
The present invention relates generally to a method for optimally allocating channel power in a W-CDMA (Wideband Code Division Multiple Access) WLL (Wireless Local Loop) system, and in particular, for allocating required channel power so as to optimize a radio capacity and a coverage of a W-CDMA WLL system.
2. Description of the Related Art
In the CDMA system, reverse channels include an access channel and a traffic channel. The access channel is used when a mobile station starts communication with a base station, or when the mobile station sends a response to a paging channel message received from the base station. An IS-95 system has 32 access channels at a maximum, for one forward paging channel, and the access channels have a data rate of 4800 bps. The reverse traffic channel operates at 4 variable data rates, like the forward traffic channel. The access channels and the traffic channels are each separated by unique user long-period codes.
Further, in the CDMA system, forward channels include a pilot channel, a sync channel, a maximum of 7 paging channels and a maximum of 63 traffic channels.
The pilot channel is used when the mobile station acquires timing for the forward channels. That is, the pilot channel provides a phase reference for performing coherent demodulation. In addition, the pilot channel provides the mobile station with the means for comparing the strength of signals transmitted from the adjacent base stations to determine whether to perform a handoff. The sync channel operates at a data rate of 1200 bps and transmits a sync message to the mobile station to synchronize the reference time between the base station and the mobile station. Further, the sync channel provides information about a bit rate of the paging channel to enable the mobile station to accurately demodulate the paging channel.
The paging channel is used for transmitting control information such as a system parameter and paging a specific mobile station. Upon the receipt of an audio signal having variable data rates of 9600, 4800, 2400 and 1200 bps, the traffic channel multiplexes signaling information and transmits the multiplexed signaling information.
FIG. 1 shows the structure of a general reverse traffic channel. As illustrated, the reverse traffic channel (R-TCH) includes 20 ms frames, each of which is comprised of 16 1.25 ms power control groups (PCGs). The user data is encoded by a coding rate r=⅓, constraint length K=9 convolutional encoder 10, repeated by a repeater 11 according to data rates, interleaved by an interleaver 12, and then orthogonally modulated by a 64-ary orthogonal modulator 13. The reverse traffic channel transmits the data only for a selected power control group at the same power, when the data rate is not 9600 bps. The reverse traffic channel intermittently transmits the data according to the data rates, and the locations are determined by a data burst randomizer 15 depending on the unique long-period PN codes of the respective users. Thereafter, the output of the data burst randomizer 15 is scrambled with a long-period PN code having a period of 242xe2x88x921 at a rate of 1.2288 MHz, and then OQPSK (Offset Quadrature Phase Shift Keying) modulated with a pair of two short-period pilot PN codes.
FIG. 2 shows the structure of a general forward traffic channel. As illustrated, the forward traffic channel includes 20 ms frames, each of which is comprised of 16 1.25 ms power control groups. The user data is encoded by an r=xc2xd, K=9 convolutional encoder 20, repeated by a repeater 21 according to data rates, and then interleaved by an interleaver 22. Thereafter, the user data is scrambled with a long-period PN code, and multiplied by one of the specified 64 Walsh codes to be spread into a signal having a rate of 1.2288 Mcps. Then, the signal is multiplied by a pair of two PN codes and undergoes the QPSK modulation before transmission.
Meanwhile, in the conventional CDMA WLL system, the base station and the mobile station cannot perform the proper power allocation of the radio channels and thus can not satisfy the performance requirements of the respective radio channels. Therefore, it is difficult to facilitate the optimization of the radio capacity and coverage of the system, thereby causing an unbalance of the forward coverage and the reverse coverage.
It is, therefore, an object of the present invention to provide a method for satisfying the performance requirements of respective channels, maximizing the radio capacity and coverage of the system and maintaining the balance of a forward coverage and a reverse coverage, in a W-CDMA WLL system.
To achieve the above object, there is provided a method for allocating reverse and forward channel power in a W-CDMA WLL system. The method includes the steps of calculating the propagation path loss of forward and reverse pilot channels depending on a maximum path loss of forward and reverse links for a base station located at a specific distance from a terminal, determined from a minimum Ec/It required by the system, a transmission antenna gain of a terminal reception antenna gain of the terminal, a reverse frequency reusing efficiency, a transmission output of the terminal, a number of active users, a power allocation ratio of the pilot channel, a bandwidth, and a thermal noise density; and, calculating the power allocation ratio of forward and reverse channels depending on the propagation path loss of the forward and reverse pilot channels, Eb/Nt of the forward and reverse channels, the power allocation ratio of the forward and reverse pilot channels, the data rate of the forward and reverse channels, the bandwidth, and the reverse frequency reusing efficiency.